JAMB 2010 · UTME · Q25

Matrices & determinants

If $\begin{vmatrix} x & 3 \\ 2 & 7 \end{vmatrix} = 15$ , find the value of $x$ .

Worked solution (try it first)

A $2 \times 2$ determinant is $ad - bc$ : $7x - 3 \times 2 = 7x - 6$ .
So $7x - 6 = 15$ .
Add 6 to both sides: $7x = 21$ .
Divide by 7: $x = 3$ , option A.

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